U.S. patent number 10,627,335 [Application Number 15/744,695] was granted by the patent office on 2020-04-21 for structure for use in infrared spectroscopy and infrared spectroscopy method using same.
This patent grant is currently assigned to RIKEN. The grantee listed for this patent is RIKEN. Invention is credited to Atsushi Ishikawa, Takuo Tanaka.
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United States Patent |
10,627,335 |
Tanaka , et al. |
April 21, 2020 |
Structure for use in infrared spectroscopy and infrared
spectroscopy method using same
Abstract
In an embodiment of the present disclosure, a structure
comprising an infrared ray absorption surface is provided for use
in infrared spectroscopy, and the infrared ray absorption surface
absorbs infrared rays in a detection wavelength range that covers
the response wavelengths of a substance to be detected. The
structure comprises, for example, a metal substrate having a metal
surface, metal components arranged at positions facing the metal
surface, and a support part that supports each of the metal
components relative to the metal surface, and a resonator for the
infrared ray absorption surface is formed by the metal substrate,
the metal components and the support parts. Additionally, in an
embodiment of the present disclosure, an infrared spectroscopy
method wherein a specimen is brought into close contact to the
infrared ray absorption surface of the structure is also
provided.
Inventors: |
Tanaka; Takuo (Saitama,
JP), Ishikawa; Atsushi (Saitama, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
RIKEN |
Saitama |
N/A |
JP |
|
|
Assignee: |
RIKEN (Saitama,
JP)
|
Family
ID: |
57756973 |
Appl.
No.: |
15/744,695 |
Filed: |
July 7, 2016 |
PCT
Filed: |
July 07, 2016 |
PCT No.: |
PCT/JP2016/070199 |
371(c)(1),(2),(4) Date: |
January 12, 2018 |
PCT
Pub. No.: |
WO2017/010411 |
PCT
Pub. Date: |
January 19, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180202918 A1 |
Jul 19, 2018 |
|
Foreign Application Priority Data
|
|
|
|
|
Jul 13, 2015 [JP] |
|
|
2015-139701 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N
21/554 (20130101); G01N 21/01 (20130101); G01N
21/35 (20130101); G01N 21/3581 (20130101); G01N
2021/3595 (20130101) |
Current International
Class: |
G01N
21/01 (20060101); G01N 21/552 (20140101); G01N
21/3581 (20140101); G01N 21/35 (20140101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Hou-Tong Chen, "Active terahertz metamaterial devices",Nov. 20,
2006, nature, vol. 444 (Year: 2006). cited by examiner .
Kai Chen, "Dual-Band Perfect Absorber for Multispectral
Plasmon-Enhanced Infrared Spectroscopy",ACSNano, vol. 6 2012 (Year:
2012). cited by examiner .
Alici, "Hybridization of Fano and Vibrational Resonances in
Surface-Enhanced Infrared Absorption Spectroscopy of Streptavidin
Monolayers on Metamaterial Substrates," IEEE Transactions on
Nanotechnology 13(2):216-221, 2014. cited by applicant .
Brown et al., "Surface-Enhanced Infrared Absorption Using
Individual Cross Antennas Tailored to Chemical Moieties," J. Am.
Chem. Soc. 135:3688-3695, 2013. cited by applicant .
Chen et al., "Uniaxial-isotropic Metamaterials by Three-Dimensional
Split-Ring Resonators," Adv. Optical Mater. 3:44-48, 2015. cited by
applicant .
International Search Report and Written Opinion, dated Oct. 4,
2016, for International Application No. PCT/JP2016/070199, 15
pages. (with English Translation). cited by applicant .
Ishikawa et al., "Background-suppressed surface-enhanced molecular
detection by metamaterial infrared absorber," Proceedings of SPIE
9163:91632Q, 2014. (5 pages). cited by applicant .
Ishikawa et al., "Negative Magnetic Permeability in the Visible
Light Region," Physical Review Letters 95:237401, 2005. (4 pages).
cited by applicant .
Osawa et al., "Surface-Enhanced Infrared Absorption of
p-Nitrobenzoic Acid Deposited on Silver Island Films: Contributions
of Electromagnetic and Chemical Mechanisms," J. Phys. Chem.
95:9914-9919, 1991. cited by applicant .
Sondergaard et al., "Plasmonic black gold by adiabatic nanofocusing
and absorption of light in ultra-sharp convex grooves," Nature
Communications 3969, 2012. (6 pages). cited by applicant.
|
Primary Examiner: Smith; Maurice C
Attorney, Agent or Firm: Seed IP Law Group LLP
Claims
The invention claimed is:
1. A structure for use in infrared spectroscopy having an infrared
ray absorption surface configured to absorb infrared rays in a
detection wavelength range covering a response wavelength of a
substance to be detected, comprising: a metal substrate having a
metal surface that provides the infrared ray absorption surface;
metal components disposed at positions facing the metal surface;
and a support part supporting each of the metal components relative
to the metal surface, wherein the support part has a material and
thickness that are configured such that the structure forms a
resonator for the detection wavelength range, the resonator
utilizing a phase shift and a phase retardation between electric
polarization in the metal substrate and electric polarization in
the metal components, and wherein the support part is formed of the
infrared transmission layer that has been patterned so as to match
shapes of the metal components.
2. The structure according to claim 1, wherein the metal components
are formed of a patterned metal film.
3. The structure according to claim 1, wherein a metal material of
the metal components includes at least any of the metal group
consisting of gold, silver, copper, aluminum, and platinum.
4. The structure according to claim 1, wherein the metal components
are linear ribbons, and the metal components form a pattern of
linear ribs which is a set of the same type of ribbons.
5. The structure according to claim 1, wherein the metal components
are island-shaped elements that are isolated from each other.
6. The structure according to claim 1, wherein the metal components
are manufactured by photolithography or electron beam
lithography.
7. The structure according to claim 1, wherein the metal components
include metal components that are assembled or arranged by a
self-assembling action.
8. The structure according to claim 1, wherein the metal substrate
includes at least any of the metal group consisting of gold,
silver, copper, aluminum, and platinum.
9. The structure according to claim 1, wherein the support part is
an infrared transmission layer that allows infrared rays in the
detection wavelength range to pass there through.
10. The structure according to claim 1, wherein the substance to be
detected has at least two response wavelengths including first and
second response wavelengths different from each other, and the
detection wavelength range includes a first detection wavelength
range covering the first response wavelength, and a second
detection wavelength range covering the second response
wavelength.
11. A method of infrared spectroscopy for a substance to be
detected, comprising steps of: providing a structure for use in the
infrared spectroscopy, which has an infrared ray absorption surface
configured to absorb infrared rays in a detection wavelength range
covering a response wavelength of the substance to be detected,
wherein the structure has a metal substrate having a metal surface
that provides the infrared ray absorption surface, metal components
disposed at positions facing the metal surface, and a support part
supporting each of the metal components relative to the metal
surface, wherein the support part has a material and thickness that
are configured such that the structure forms a resonator for the
detection wavelength range, the resonator utilizing a phase shift
and a phase retardation between electric polarization in the metal
substrate and electric polarization in the metal components, in
such a state that a specimen potentially containing the substance
to be detected is brought into close contact with the infrared ray
absorption surface and the support part is formed of the infrared
transmission layer that has been patterned so as to match shapes of
the metal components; irradiating the infrared ray absorption
surface with the infrared rays in the detection wavelength range;
and detecting an intensity spectrum of reflected infrared rays from
the infrared ray absorption surface.
12. The method according to claim 11, wherein the irradiating step
is a step of irradiating the infrared ray absorption surface with
the infrared rays from an incidence direction inclined to the
surface, and the detecting step is a step of detecting a reflection
peak at a wavelength corresponding to the response wavelength in
the detection wavelength range for the reflected infrared rays.
13. The method according to claim 12, further comprising a step of:
determining at least any of a presence/absence, a component
content, a type, a chemical structure, and ambient information
regarding the substance to be detected in the specimen, based on
the reflection peak which corresponds to the substance to be
detected and appears in an intensity spectrum of the reflected
infrared rays.
Description
BACKGROUND
Technical Field
The present disclosure relates to a structure for use in infrared
spectroscopy, and to the infrared spectroscopy using the same. More
specifically, the present disclosure relates to the above structure
which is suitable for detecting a substance to be detected in a
specimen by the infrared spectroscopy, and to the infrared
spectroscopy using the structure.
Description of the Related Art
Infrared spectroscopy occupies an important position in materials
science, medical science, and security detection technology. In the
infrared spectroscopy, by using the properties that a substance
absorbs infrared rays having a specific wavelength (wavenumber)
peculiar to the substance, any of the presence/absence, the
component content, the type, the chemical structure, and the
ambient information for a substance of interest (substance to be
detected: test material) in the specimen is determined. This is
because this absorption is usually associated with molecular
vibration, and dominant information with respect to the molecular
structure, the composition, and the ambient condition is reflected
in the molecular vibration. In typical infrared spectroscopy, the
infrared rays after having been transmitted through or reflected
from the specimen are compared with appropriate reference
conditions. Then, the substance to be detected is quantified by a
process of determining whether or not absorption peculiar to the
substance to be detected is observed in the specimen, by precisely
determining the absorption wavelength, or by using the absorption
quantity as a clue. For a rapid and convenient infrared inspection
technology in various applications, it is preferable that such
technology can directly detect as small amount as possible of
molecules through far-field measurement rather than proximity
measurement that adopts a probe or the like.
On the other hand, light absorption, which is one of the most
fundamental light-matter interactions, is also an essential
phenomenon for a variety of the optical application fields, such as
photovoltaic cells and thermal management. Materials having a large
absorption constant may exhibit strong light absorption, but they
often show strong reflection due to a large impedance mismatching
at the interface with other materials. In response to this mismatch
at the interface, it has been tried to artificially adjust or
tailor resonance and dispersion of material, by adopting artificial
substances including metallodielectric nanostructures that has been
emerging as one of plasmonic metamaterials. In the field of the
plasmonic metamaterials, a new degree of freedom has been
introduced by manipulating the optical field at a nano- to
macroscopic scale, whereby an attractive technology for sensing
applications has been developed. The most important feature among
such capabilities is the artificial controllability of two separate
macroscopic optical properties of refractive index and
characteristic impedance, which makes it possible to control light
to an ultimate level.
In the above described infrared spectroscopy, as means for
measuring the infrared spectrum with a high sensitivity for a very
small amount of the object specimen, surface-enhanced infrared
absorption (SEIRA) spectroscopy using a mirror such as a metal thin
film has been vigorously studied (for instance, Patent Literature
1). In SEIRA, a metal surface having fine particles of nanometer
order thereon is manufactured, and object molecules to be detected
are adsorbed to a metal thin film composed of the above fine
particles. In fact, a dramatic improvement of the sensitivity by
several orders of magnitude has been demonstrated by tailored
plasmonic nanostructures (Non-Patent Literature 1). Currently,
efforts to improve the detection sensitivity in infrared
spectroscopy using the metamaterials are focused on hot-spot
engineering that pursues a phenomenon in which electromagnetic
fields are enhanced in between fine particles, in the vicinity of a
corner portion, and the like (for instance, Non-Patent Literature
2). Furthermore, as a result of recent development of a
metamaterial absorber, strong or even perfect absorption is
achieved within a certain frequency range (Non-Patent Literature
3). In the metamaterial absorber, unique surface conditions are
given in which the absorption characteristics associated with
intense plasmonic enhancement are adjusted, and accordingly various
potential applications such as a high-efficiency thermal radiator
and high-sensitive bio-chemical sensing have been proposed.
CITATION LIST
Patent Literature
Patent Literature 1: Japanese Patent Laid-Open No. 2009-080109
Non Patent Literature
Non Patent Literature 1: M. Osawa and M. Ikeda, "Surface-enhanced
infrared absorption of p-nitrobenzoic acid deposited on silver
island films: contributions of electromagnetic and chemical
mechanisms," J. Phys. Chem. 95, 9914 (1991).
Non Patent Literature 2: L. V. Brown, et al. "Surface-enhanced
infrared absorption using individual cross antennas tailored to
chemical moieties," J. Am. Chem. Soc. 135, 3688 (2013).
Non Patent Literature 3: T. Sondergaard, et al. "Plasmonic black
gold by adiabatic nanofocusing and absorption of light in
ultra-sharp convex grooves," Nat. Commun. 3, 969 (2012).
Non Patent Literature 4: A. Ishikawa, T. Tanaka, and S. Kawata,
"Negative Magnetic Permeability in the Visible Light Region," Phys.
Rev. Lett. 95, 237401 (2005). 10.1103/PhysRevLett.95.237401
Non Patent Literature 5: C.-C. Chen, A. Ishikawa, Y.-H. Tang, M.-H.
Shiao, D. P. Tsai, and T. Tanaka, "Uniaxial-isotropic Metamaterials
by Three-dimensional Split-Ring Resonators," Adv. Opt. Mater. 3,
pp. 44-48 (DOI: 10.1002/adom.201400316) (2015).
BRIEF SUMMARY DISCLOSURE
Technical Problem
The universal problem of the infrared spectroscopy which obtains
various information about the substance to be detected such as
molecules is that it is difficult to quickly and simply measure the
substance to be detected under such conditions that the signal is
weak, in particular, that the specimen is an extremely small
amount. As a countermeasure against this problem, in SEIRA, it is
tried to utilize an electric field enhancement due to superposition
of the optical electric fields that are produced by a mirror. In
advanced SEIRA, a technique that may be referred to as hot-spot
engineering attracts attention, in which gaps between fine
particles are used and positions (hot spots) are created where an
electromagnetic field is strengthened locally. A technology which
has recently been investigated intensively in order to aim for a
level of attomole to zeptomole (10.sup.-18 to 10.sup.-21 moles) is
the above described hot-spot engineering; and is the spatial
overlapping of near-field enhancement between plasmons and
molecular vibrations, and the overlapping of wavelength modes.
However, since the SEIRA is a technique of finding out a lowered
reflectivity portion caused by the enhanced infrared absorption, a
signal indicating the lowered portion appears in bright reflection
light (background light) reflected from a metal thin film. As a
result, the most part of such bright reflection light directly gets
into the detector, which becomes problematic stray light. When the
absorption is weak, the above described stray light and relatively
bright background light form noises by themselves, which makes it
difficult to obtain a high signal-to-noise ratio.
In addition, in the SEIRA, the degree of enhancement of the
obtained signal greatly depends on the nano-level structure of the
metal surface. For instance, when particles are used, the narrower
is the spacing between the particles, the more strongly is the
electric field enhanced. However, it is not easy to efficiently
manufacture and evaluate such a metal structure of nanometer order
(for instance, Patent Literature 1). Not only in the case where the
thin films of the metal structure are different from each other,
but also in the case within one film, a problem arises in
reproducibility, because degree of absorption in the infrared range
tends to change depending on the observed region. In reality,
although the SEIRA provides a considerably large signal intensity
in practical applications, it is true that the detection of
picomole (10.sup.-12 moles) level specimens (in other words, of
monomolecular film) still remains challenging.
Solution to Problem
The present inventors have found that a significant improvement of
the sensitivity based on a novel principle will be yielded in the
field of infrared spectroscopy, by using a material body
(structure) having an infrared ray absorption surface in the
infrared spectroscopy. Specifically, in an aspect of the present
disclosure, a structure for use in infrared spectroscopy will be
provided which has an infrared ray absorption surface configured to
absorb infrared rays in a detection wavelength range covering the
response wavelength of a substance to be detected.
In another aspect of the disclosure, a method of an infrared
spectroscopy for a substance to be detected is also provided, which
includes steps of: providing a structure for use in the infrared
spectroscopy, which has an infrared ray absorption surface
configured to absorb infrared rays in a detection wavelength range
covering a response wavelength of the substance to be detected, in
such a state that a specimen potentially containing the substance
to be detected is brought into close contact with the infrared ray
absorption surface; irradiating the infrared ray absorption surface
with the infrared rays in the detection wavelength range; and
detecting an intensity spectrum of reflected infrared rays from the
infrared ray absorption surface.
In any of the aspects of the present disclosure, the infrared ray
absorption surface provided in the above described structure is
used. The infrared ray absorption surface is a surface that is
configured to absorb rays in a certain wavelength range out of the
emitted infrared rays, to some extent. In the first place the above
infrared ray absorption surface of the above described structure is
manufactured so that the response wavelength of the substance to be
detected falls within the wavelength range, and thereafter the
infrared rays for detecting the substance to be detected are
emitted to irradiate the infrared ray absorption surface. When
detection of reflected infrared rays is conducted on the infrared
ray absorption surface, the resulting component in the
above-mentioned wavelength range should be reduced, and accordingly
a lowered background is provided. This leads to the reduction of
the infrared rays which may well become stray light for an infrared
ray detector. Since the wavelength range in which the infrared ray
absorption surface absorbs the infrared rays is a detection target
of the infrared spectroscopies that will be achieved in each aspect
of the present disclosure, it is therefore particularly referred to
as a detection wavelength range in the present application.
In the structure having the above described features, when the
substance to be detected is positioned very close to the infrared
ray absorption surface, both of the infrared ray absorption surface
and the substance to be detected respond to the infrared rays, in
particular, corresponding to the response wavelength out of the
detection wavelength range. Because of this, the infrared ray
absorption characteristics for the infrared ray absorption surface
are disturbed by the response of the substance to be detected. Such
disturbance has an effect of weakening the absorption typically by
the infrared ray absorption surface, and accordingly under suitable
conditions, the peak where increased intensity of the infrared ray
is observed should be found at a wavelength that is substantially
same as the above described response wavelength. Since this change
in the infrared spectrum is sensitive, which originates in a
phenomenon that the substance to be detected disturbs the low
reflection condition of the infrared ray absorption surface,
detection with high sensitivity at a high signal-to-noise ratio is
achieved in combination with the background reduced by the
absorption. Additionally, when the background producing noise is
reduced, it also becomes unlikely that saturation takes place even
in the case where the detector is operated to accumulate signals.
Also from this point of view, both of the above described aspects
are highly practical.
In the above described aspects of the present disclosure, it is
particularly advantageous that the absorption on the infrared ray
absorption surface of the structure is generated as a result of the
resonance of the resonator. If a technology referred to as
so-called metamaterial or metasurface is applied, the resonance can
be occurred in a target frequency range, by an operation of
tailoring the response to temporal vibration in an electric field
or a magnetic field of the infrared rays. The electromagnetic
resonant frequency and its width can be tailored in the structure,
by placing metal components with a designed size and shape on the
surface of the structure or in the vicinity of the surface, which
makes it possible to adapt the detection wavelength range to the
target response wavelength of the objective substance to be
detected. The resonance between this electromagnetic field
(infrared ray, or light) and the structure localizes the
electromagnetic energy originating in the infrared rays onto the
infrared ray absorption surface. The localized electromagnetic
energy is multiply absorbed in the vicinity of the same surface
according to the Q value of this resonance, and accordingly the
absorption efficiently occurs, which leads to the energy
dissipation in a form of loss. When the infrared ray absorption
surface is properly tailored, an energy fraction to be re-emitted
in the form of the infrared rays out of the electromagnetic field
oscillating in the detection wavelength range becomes very weak. In
the above described aspect of the present disclosure, the infrared
rays of the response wavelength of the substance to be detected or
the wavelength in the vicinity thereof are covered in the detection
wavelength range. Therefore the infrared rays cause not only the
resonance between the light and the structure, but also the
resonance between the electromagnetic field (or light) and the
molecular vibrations. Since the resonance between the light and the
molecular vibrations acts also on the absorption due to the
resonance between the light and the structure, its influence
appears on the absorption spectrum of the infrared ray absorption
surface, which is finally detected. The interaction between the
phenomena originating in this resonance is particularly sensitively
detected in the structure in which the infrared ray absorption
surface is based on the principle of the resonance.
The infrared ray in the present application is an electromagnetic
wave which is generally included in a wavelength region, for
instance, of 1 .mu.m to 1 mm (frequency region of 300 GHz to 300
THz). Among the infrared rays, the above infrared ray includes a
wavelength range suitable for infrared spectroscopy, specifically,
a far-infrared to mid-infrared region or a THz wave region
(wavelength region of 20 .mu.m to 600 .mu.m: and frequency region
of 0.5 THz to 15 THz), and also include a wavenumber region of 650
to 1300 cm.sup.-1 (wavelength region of 7.7 .mu.m to 15.4 .mu.m, or
frequency range of 19.5 THz to 39 THz), in particular, which is
also referred to as a finger print region. In the present
application the terminology of the conventional use in the
technological field of the present disclosure is used, so long as
it does not render the description ambiguous. For instance, it
occasionally occurs to use expressions which are used in optical
fields such as "light", "light source", "light emission",
"transmitted light", "reflection light", for electromagnetic waves
other than visible light, such as electromagnetic radiation in the
infrared region. Therefore, the electromagnetic wave in the
infrared ray region may also be referred to as infrared light.
Furthermore, when the infrared rays are characterized according to
the conventional use of infrared spectroscopy, nomenclatures or
terminologies on the wavelength region, the wavenumber range or the
frequency range are used. For instance, when the frequency is
expressed, it occasionally occurs to use a numerical value and a
unit of the wavenumber for easiness of understanding, and show the
numerical value of the wavenumber in such a manner that the value
decreases toward the right on the axis of the graph. In addition,
it also occurs to describe the numerical value and the unit of
frequency, while expressing the wavelength, in some cases. These
values are mutually converted in accordance with the dispersion
relationship of f=c/.lamda. (where f denotes frequency, .lamda.
wavelength in vacuum, and c the velocity of light), which also
holds for ordinary infrared rays. It means that the detection
wavelength range, for instance, should be understood not only as
the range of the wavelength, but also as the range of the frequency
and the range of the wavenumber which have been converted. The
response wavelength should be also similarly understood to the
above description. The wavenumber in the description of the present
application is defined by the inverse number (1/.lamda.) of the
wavelength, is described usually by using cm.sup.- as the unit, and
is different from the wavenumber of the angle unit defined by a
factor of 2.pi./.lamda.. Furthermore, as for the wavelengths and
the wavenumbers, the numerical value in the vacuum is occasionally
used, even though the electromagnetic wave is transmitted through a
medium having a refractive index larger than 1, in accordance with
the custom in the field of optical technology. The response
wavelength of the substance to be detected generally refers to the
wavelength of infrared rays that cause interaction due to molecular
vibrations or the like. Therefore, the wavelength is expressed as
the response wavelength, at which the absorption due to the
substance to be detected is observed if the substance to be
detected is measured in the ordinary infrared spectroscopy; and
therefore the infrared ray having the response wavelength may not
always be absorbed for any implementation of the aspects in the
present application. This response wavelength reflects the features
of the substance to be detected, accordingly there is a case where
one or more response wavelengths are associated with a single
substance to be detected; there is also a case where the response
wavelength has a certain wavelength width originating in the
molecular vibrations; and there is also a case where the response
wavelength degeneration is observed due to the symmetry of the
molecular structure and the like.
Advantageous Effect of the Disclosure
In any of the aspects of the present disclosure, the infrared
spectroscopy will be achieved which can detect the substance with
high sensitivity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing a schematic configuration of a
structure for typical infrared spectroscopy in an embodiment of the
present disclosure.
FIGS. 2A, 2B and 2C are explanatory views each showing a principle
of the infrared spectroscopy by an intensity spectrum, in the
embodiment of the present disclosure.
FIGS. 3A and 3B are schematic views each illustrating a structure
of a metamaterial infrared absorber having linear ribs, which is a
typical example of the embodiment of the present disclosure; and
are a perspective view (FIG. 3A) showing a whole structure and an
enlarged sectional view (FIG. 3B) showing only one unit of
individual ribbons having a repeating structure.
FIG. 4 is a flowchart showing analysis processing by the infrared
spectroscopy in the embodiment of the present disclosure.
FIGS. 5A and 5B are an external photograph (FIG. 5A) and an SEM
image (FIG. 5B) of an example sample of a structure, which is a
metamaterial absorber that has been manufactured in an example of
the embodiment of the present disclosure.
FIGS. 6A and 6B are reflectance maps showing the characteristics of
infrared reflectance in the structure of the embodiment of the
present disclosure, which has a structure shown in FIG. 3, as a
function of frequency (vertical axis) and incident angle
(horizontal axis); and experimental values (FIG. 6A) and a
numerical simulation result (FIG. 6B) in the example sample are
shown in contrast to each other.
FIG. 7 shows calculation results of the electromagnetic field
distribution of each mode in the example sample of the present
disclosure.
FIGS. 8A and 8B are graphs that illustrate actually measured data
which were obtained in the example sample of the present
disclosure; and FIG. 8A shows the data which were measured values
of the reflectance spectrum obtained from a SAM of 16-MHDA, at
.theta.=80.degree. on a bare Au surface (upper), and the data which
were measured values at different incident angles from 30.degree.
to 70.degree. on the metal surface of the structure (lower).
In addition, FIG. 8B is a spectral curve shape (upper) of the
vibrational signals at incident angle .theta.=40.degree. and the
experimental result which were extracted, and a Fano fitting curve
(lower) for reproducing the same curve shape using corresponding
parameters.
FIGS. 9A and 9B are perspective views illustrating another infrared
ray absorption surface according to the embodiment of the present
disclosure; and are an example in which an array of SSRRs is formed
for the infrared ray absorption surface (FIG. 9A), and an example
in which an array of 3D-SRR is formed (FIG. 9B).
FIG. 10 is a schematic diagram showing an example of a structure
that corresponds to a plurality of detection wavelength ranges, in
the embodiment of the present disclosure.
FIG. 11 has a schematic view (FIG. 11A) showing another example of
the structure that corresponds to a plurality of detection
wavelength ranges, in the embodiment of the present disclosure; and
a perspective view (FIG. 11B) showing a structure in which the
metal component forms a plurality of types of shapes, as a specific
example of the above structure, in the embodiment.
DETAILED DESCRIPTION
The structures for use in the infrared spectroscopy according to
the present disclosure and the embodiments of the infrared
spectroscopy using the structures will be described below. Common
portions or elements will be denoted by common reference
characters, unless otherwise particularly referred to in the
description. In addition, in the figures, each of the elements in
each embodiment is not necessarily shown to scale.
1. Infrared Spectroscopy Using Infrared Ray Absorption Surface
In the present embodiment, an infrared ray absorption surface is
used for the infrared spectroscopy. The infrared ray absorption
surface can be achieved based on a metamaterial technique. The
types thereof are: a metal substrate having a metal surface; metal
components that are disposed at positions facing the metal surface;
and a surface of the structure having a support part which supports
each of the metal components relative to the metal surface. In this
structure, the metal substrate, the metal components, and the
support part form a resonator that exhibits resonance to thereby
achieve the infrared ray absorption.
FIG. 1 is a perspective view showing a schematic configuration of a
structure 100 (hereinafter referred to as "structure 100") for
typical infrared spectroscopy of the present embodiment. The
structure 100 has an infrared ray absorption surface 20. The
infrared ray absorption surface 20 is manufactured so as to absorb
at least infrared rays in a detection wavelength range covering a
response wavelength of a substance to become an object to be
detected (substance to be detected) to some extent, in other words,
so as to have a dip D on the reflection spectrum. The substance to
be detected is included in a specimen (not shown), and the specimen
is disposed so as to come into close contact with the infrared ray
absorption surface 20. The infrared rays I.sub.i incident on the
structure 100 are generally incident on the infrared ray absorption
surface 20 at a predetermined incident angle .theta.. At the time
when the infrared rays I.sub.i are emitted, the specimen attaches
to or is adsorbed on the surface of the infrared ray absorption
surface 20 of the structure 100. In the structure 100 itself and
the space facing the infrared ray absorption surface 20, ambient
conditions such as an atmosphere (type of gas, and pressure) and
temperature are appropriately arranged as needed. The incident
infrared ray I.sub.i is an infrared beam that is provided through
an appropriate light source or optics (not shown), and the
specularly reflected reflection light I.sub.o is detected by an
appropriate light receiving optics or detector. Wavelength
resolution for determining the intensity spectrum is achieved by
physical means or calculation means, for the incident infrared rays
I.sub.i beforehand or for the reflection light I.sub.o afterwards.
In one example, the wavelengths of the infrared rays I.sub.i are
resolved by a diffraction grating beforehand, and then the resolved
infrared rays irradiate the infrared ray absorption surface 20.
Another technique is a technique of using the FT-IR spectroscopy,
which includes: dividing the reflection light I.sub.o reflected
from the infrared ray absorption surface 20 into two arms by an
interference optics; making the light of one arm interfere with the
light of the other arm while scanning the optical path length of
the one arm; and acquiring the intensity spectrum as a power
spectrum by autocorrelation. In such an infrared ray absorption
surface 20, the structure 100 itself functions as a metamaterial
absorber (or a metasurface absorption surface), and brings the dip
D to the detected infrared reflection spectrum.
FIG. 2 is an explanatory view showing a principle of infrared
spectroscopy in the present embodiment, by the intensity spectrum.
FIGS. 2A and 2B are schematic views of the reflection spectrum on
the infrared ray absorption surface 20 of the structure 100, and a
schematic view of an absorption spectrum of the substance to be
detected that can be contained in the specimen, respectively.
Furthermore, FIG. 2C shows a reflection spectrum of the infrared
rays reflected on the infrared ray absorption surface 20, when the
specimen containing the substance to be detected exists so as to
come into close contact with the infrared ray absorption surface
20. Abscissa axes of each graph are made to match with each other.
In general, the resonance occurs between light and the structure on
the infrared ray absorption surface, thereby the localization of
the electromagnetic energy corresponding to the Q value is
achieved. The dip D of the reflectance on the infrared ray
absorption surface due to the metamaterial absorber, or the
detection wavelength range, becomes to have a relatively wide width
in wavelength, as compared with the narrow spectral line width in
the vicinity of the response wavelength of the substance to be
detected. Thus, in the typical measurement of the present
embodiment, the response wavelength of the substance to be detected
can be included in the detection wavelength range which becomes a
low background. The detection wavelength range is observed as the
dip D (FIG. 2A) in which the reflectance of the infrared ray
absorption surface 20 is suppressed, while covering at least one of
the response wavelengths of the substance to be detected. The dip D
in the reflection spectrum directly gives the detection wavelength
range, and accordingly is hereafter referred to also as the
detection wavelength range D by the same reference character. When
the specimen containing the substance to be detected exists so as
to come into close contact with the infrared ray absorption surface
20, a peak is observed at the same or substantially the same
wavelength, in the reflection spectrum (FIG. 2C), as the response
wavelength (FIG. 2B) of the substance to be detected. Since this
peak can be explained by Fano's theory that describes an
interaction between a plurality of resonance phenomena, as will be
described later in examples, it is considered that such interaction
should be found for two resonances; one is resonance responsible
for the absorption in the infrared ray absorption surface 20
(resonance between light and structure), and the other is one
originated in the molecular vibrations at the response wavelength
of the substance to be detected (resonance between light and
molecular vibration). In the detection wavelength range D, the
background light is suppressed, which improves sensitivity further.
Therefore, the technique of the present embodiment is an attractive
approach for acquiring a significant signal-to-noise ratio. The
reflectance on the infrared ray absorption surface 20 in the
wavelength region deviating from the detection wavelength range D
is not particularly limited. Such a phenomenon is achieved when the
specimen comes into close contact with the infrared ray absorption
surface 20, and the substance to be detected in the specimen also
comes into close contact with the infrared ray absorption surface
20. A typical example is the case where the substance to be
detected microscopically comes into contact with the infrared ray
absorption surface 20, for instance, by chemical adsorption or
physical adsorption. However, as long as the peak is observed as
has been described in FIG. 2C, the form and the distance of the
close contact between the substance to be detected and the infrared
ray absorption surface 20 are not limited in the present
embodiment.
It is to be noted, in the conventional technique, that absorption
by the substance to be detected may be measured in transmitted or
reflected infrared rays. Absorption originates in phenomenon in
which energy is transferred from the emitted infrared rays to the
molecular vibrations of the substance to be detected. This
principle also holds for conventional SEIRA, which attempts to
increase the interaction between light and the substance to be
detected, by hot-spot engineering which locally enhances the
electric field and the magnetic field on the surface of the
reflection plane, and to enhance the measurement sensitivity for
the absorption by the molecular vibrations. Accordingly, in any
case, the conventional technique attempts to find absorption in
signals of comparatively strong background light. Thus, as far as
the conventional technique is concerned, such a tendency is common
that as the content of the substance to be detected decreases, the
absorption signal tends to be buried because of a strong
background. Furthermore, it can be assumed that the peak shown in
FIG. 2C does not occur even though the knowledge of the ordinary
infrared spectroscopy has been simply applied to the situation in
FIG. 2. Even if the molecular vibration of the substance to be
detected has shown the absorption as in FIG. 2B based on the
resonance between the light and the molecular vibration, factors
that may be responsible for is merely absorption phenomena occurred
in a multiple manner.
2. Elements of Structure for Achieving Infrared Ray Absorption
Surface
In the present embodiment, various types are considered as such a
structure of the structure 100 as to achieve the infrared ray
absorption surface 20 of FIG. 1, and one typical example is a
structure in which a large number of ribbons having a
metal-insulator-metal (MIM) layered structure are repeatedly
aligned so as to form linear ribs of a one-dimensional array. FIGS.
3A and 3B are views that show a structure 102, which is one typical
example of the structure 100 and is a metamaterial infrared
absorber having linear ribs; and are a perspective view (FIG. 3A)
showing a whole structure of the linear ribs, and an enlarged
cross-sectional view (FIG. 3B) showing only one unit of individual
ribbons which have a repeating structure and have each the MIM
layered structure. As is shown in FIG. 3B, the structure 102 has a
metal substrate 30 which has a thick Au film, and a metal component
50 which is an Au micro-ribbon existing above the metal substrate
30; and the metal substrate 30 and the metal component 50 are
separated from each other by a support part 40 which is a MgF.sub.2
gap layer. The structure 102 has an infrared ray absorption surface
22 which has unit cells having such an MIM layered structure. The
metal substrate 30 has such a structure as to have the metal layer
34 formed on one surface of an appropriate substrate 32, for
instance, of glass or the like, so as to have an enough thickness
which is thicker than a skin-depth and can neglect transmission
(for instance, if the metal layer is made of gold, 2 .mu.m or
thicker even in the case of long wavelength); and the metal layer
34 has the metal surface 36 as its surface. The metal layer 34 may
be a single metal layer having a single composition, a composite
layer having different compositions, an alloy layer or the like.
The whole metal substrate 30 may be formed of a metal foil or a
metal plate, and thereby may have the metal surface 36 formed
thereon.
The condition to be regarded as the metal surface 36 of the present
embodiment is that the surface is made from such a material as to
have a sufficient amount of carriers responsible for the conduction
of free electrons and the like, and easily respond to the
electromagnetic field of the infrared rays in the detection
wavelength range, in other words, as to exhibit a metallic
behavior. As long as the condition is satisfied, the outermost
surface of the layer of an arbitrary material and thickness can be
used as the metal surface 36. Preferably, metal including at least
any of the metal group consisting of gold, silver, copper, aluminum
and platinum is used for the metal surface. Specifically, the
typical metal surface 36 is the outermost surface of an ordinary
metal (for instance, gold) which has reflectivity shown by the
ordinary metal and absorbs incident energy that has not been
reflected. In general, materials which are not classified as metal
can be used for the metal surface 36 of the present embodiment, if
the surface is made from a material (for instance, semiconductor
doped with impurities, in particular, degenerated semiconductor or
the like) which exhibits a metallic behavior in the detection
wavelength range out of the infrared region, as long as the
material is suitable in the used wavelength region.
The metal component 50 is disposed at a position facing the metal
surface 36 when viewed from the metal substrate 30. The metal
components 50 typically form an array that occupies positions on
facing portions of the metal surface 36. The material of the metal
component 50 may be the same material as the material for the metal
surface 36 or may be a different material therefrom. In addition,
the metal component 50 can be also made from a material that
exhibits metallic behavior in the detection wavelength range out of
the infrared range. In the case where linear ribs are used which
are ribbons arranged as in the structure 102, a period .LAMBDA. and
a width w are adjusted so that the detection wavelength range D
covers the reaction wavelength of the substance to be detected.
Specifically, the material of the metal component 50 is an
arbitrary metal that easily makes a metamaterial absorber in the
detection wavelength range D, and is preferably a metal including
at least any one selected from the metal group consisting of gold,
silver, copper, aluminum, and platinum.
The support part 40 supports the metal component 50 so that the
metal component 50 is positioned away from the metal substrate 30.
The individual ribs are ribbons that form the MIM layered structure
together with the metal substrate 30. However, the support part 40
does not necessarily need to have the same plane shape as that of
the metal component 50. A typical material of the support part 40
is MgF.sub.2. In addition to MgF.sub.2, a glassy inorganic film can
also be adopted. The support part 40 forms typically a linear rib
similar to the metal component 50 as in FIG. 3A. The support part
40 which has adopted MgF.sub.2 of the present embodiment shows a
small absorbance for and has transparency for the infrared rays,
and plays a role of adjusting an electrical coupling between the
metal component 50 and the metal substrate 30, through its
dielectric characteristics. More generally, the function carried
out by the support part 40 is at least to support the metal
component 50 above the metal substrate 30; and to electrically
couple both of the substrate and the component in order to form the
structure, and to allow the infrared rays in the wavelength region
of interest to pass therethrough. The material and thickness of the
support part 40 which determines the electrostatic capacitance
between the metal component 50 and the metal substrate 30 are
adjusted so that the detection wavelength range D (FIG. 2A)
suitable for the substance to be detected can be obtained. Between
the metal component 50 and the metal substrate 30 which the support
part 40 electrostatically couples, a phase difference is produced
between both electric polarizations in the metal component 50 and
the metal substrate 30 with a phase delay (retardation effect) due
to an the thickness effect of the support part 40, which suppresses
the reflected infrared rays generation, making the incident
infrared rays efficiently absorbed in the metal substrate 30. In
this way, it is possible to implement the structure 102 of a
metamaterial absorber having an MIM layered structure with
designated detection wavelength range D.
In addition to the above described typical example, such an
arbitrary element can be adopted for the infrared ray absorption
surface 20 of the present embodiment as to be capable of achieving
an appropriate detection wavelength range D including the response
wavelength of the target substance to be detected, by tailoring the
shapes of the cross section and the flat surface and the layer
configuration. Another typical example of the cross-sectional
configuration is a configuration in which the whole surface of the
metal surface 36 is covered with a film that adopts such a material
as to allow the infrared rays to pass therethrough such as
MgF.sub.2, and only the metal component 50 is patterned. In
addition, another preferable example of the flat surface
configuration has such a configuration as to have a cross section
of the MIM layered structure of FIG. 3B, and to have rectangular
island-shaped elements having gaps not only in the x-direction but
also the y-direction, or a circular shape. Alternatively, it is
also possible to adopt a configuration in which the relationship of
the patterns of these existing sea and islands is interchanged
(i.e., negative/positive pattern inversion), when arranging an
array of rectangular or circular openings. In a flat surface shape
having a two-dimensional shape such as an island-shaped element or
opening, the alignment of the lattice shape that individual
elements form and the size of individual patterns are adjusted.
Also, a covering ratio of how large area is covered with the metal
component 50 out of the area of the metal surface 36 is adjusted.
These adjustments are made so as to make detection wavelength range
D appropriate.
The metal component 50 can be manufactured preferably by patterning
the metal film. The patterning technique is not particularly
limited, and an arbitrary manufacturing technique including other
configurations than the linear ribs pattern may be adopted. For
instance, techniques such as photolithography and electron beam
lithography can be adopted. It should be noted that the patterning
of the metal film includes not only a technique of forming the
metal film on a flat plane and then partially removing the metal
film, but also a technique of partially forming the metal film in a
positional selective manner (for instance, pattern deposition).
Also, what may be adopted is a technique of patterning a resist
film or the like, which works as a mask and removing the resist
film, such as liftoff, to thereby form also a pattern of the metal
film. Furthermore, the metal component 50 does not necessarily need
to use the metal film in order to tailor the detection wavelength
range according to the substance to be detected. For instance, it
is also preferable to manufacture a large number of metal
components for arranging them into an intended arrangement by
combining fine components such as metallic microspheres through a
self-assembling action to create metal components acting as
individual units, without using a lithographic process or with
using the lithographic process in an auxiliary way.
In the structure 102 which has been manufactured as in the above
description, the plasmonic mode is excited as a result of the
resonance between the light and the structure. If the structure of
the metal component 50 and the support part 40 is not manufactured,
and if only the metal substrate 30 is used, most of the infrared
rays incident on the metal surface are reflected, and a small
remainder should be absorbed by the metal. On the other hand, in
the structure 102, the metal component 50 and the support part 40
having such appropriate shapes as to fit the detection wavelength
range are provided. As a result, the energy of the infrared rays
incident at the frequency of the detection wavelength range is
localized in a form of the plasmonic mode in the vicinity of the
metal surface 36, due to the electromagnetic resonance among the
Au/MgF.sub.2/Au structure, or among a structure of the metal
component 50, the support part 40, and the metal substrate 30. The
energy impinging into the metal surface 36 in a form of the
infrared rays in the detection wavelength range D excites this
mode, and is transmitted into and absorbed by the media of the
metal substrate 30 without being reflected, which results in the
suppressed reflection by the metal substrate 30. When such an
infrared ray absorption surface is adopted as to have the structure
embodying the metamaterial absorber of the present embodiment, the
infrared ray absorption surface keeps the signal-to-noise ratio
significant in the far field measurement, achieves the sensitivity
at the attomole level, and can lower the detection limit of direct
infrared absorption spectroscopy. In addition, as will be described
as relating matters in the examples, an infrared detection
technique sensitive to an extremely small amount of molecules is
achieved through the unique ambient condition of the surface of the
structure 102. At this time, since the sensitivity is improved due
to its low background, the strong absorption of the structure 102
of the metamaterial absorber is distinguished in a form of a
distinctive anti-resonance peak in the detection wavelength range
D, which allows vibrational signal to be detected.
3. Analysis Processing by Infrared Spectroscopy
FIG. 4 is a flowchart showing analysis processing by infrared
spectroscopy according to the present embodiment. In the infrared
spectroscopy of the present embodiment, firstly, the above
described structure is provided (S02). The specimen to be analyzed
comes into close contact with the infrared ray absorption surface
20 of the structure. There is a case where the specimen for the
infrared ray absorption surface 20 is provided after the specimen
has been brought into close contact with the surface beforehand,
and is a case where the specimen is supplied to the infrared ray
absorption surface or is exchanged, in the middle of the processing
of the analysis by infrared spectroscopy. Then, the infrared ray
absorption surface 20 is irradiated with the infrared rays in the
wavelength range covering at least the response wavelength which
the substance to be detected shows (S04), and the intensity
spectrum of infrared rays reflected from the infrared ray
absorption surface 20 is detected (S06).
The irradiation step (S04) is typically a step of irradiating the
infrared ray absorption surface 20 with the infrared rays from an
incidence direction inclined to the surface. Furthermore, in the
detecting step (S06), the intensity of the reflected infrared rays
is detected in the specular reflection direction on the infrared
ray absorption surface. In addition, another typical of the
irradiating step (S04) is to emit the infrared rays of polarized
light whose absorption is significant on the infrared ray
absorption surface.
In addition to the above described processing (S02 to S06), a
determination step (S08) can also be further carried out which
determines at least any one of the presence/absence, the component
content, the type, the chemical structure, and the ambient
information regarding the substance to be detected in the specimen,
on the basis of the reflection peak corresponding to the substance
to be detected found in the intensity spectrum of the reflected
infrared rays. At this time, a comparison using databases such as
known substances and known chemical bonds may also be carried out
as needed.
4. Example
The present disclosure will be described more specifically with
reference to examples. Materials, amounts to be used, rates,
contents of treatment, treatment procedures and directions,
specific arrangements of elements or members and the like, which
will be described in the following examples, can be suitably
changed unless they deviate from the scope of the present
disclosure. Accordingly, the scope of the present disclosure is not
limited to the following specific examples. An example sample of
the structure 102 (FIG. 3) was actually manufactured in which the
detection wavelength range D (FIG. 2) was set in the wavelength
region which was centered on approximately 3000 cm.sup.-1. In the
step, for the metal substrate 30, firstly, a thin film of gold with
a thickness of 200 nm was vapor-deposited on an SiO.sub.2 substrate
provided with a bonding layer of 5 nm thick Cr, by electron beam
heating. Subsequently, for the purpose of forming the support part
40 and the metal component 50, an array of one-dimensional (1D)
micro-ribbons was patterned on the Au film surface by a standard
photolithography process. Specifically, a surface structure was
obtained through vapor deposition of thin films of 30 nm MgF.sub.2
and 50 nm gold and a lift-off process. FIG. 5 is an appearance
photograph (FIG. 5A) and an SEM image (FIG. 5B) of an example
sample of the structure 102 which is a manufactured metamaterial
absorber. In the manufactured example sample, a uniform surface
structure was obtained over the whole surface of the chip having as
wide an area as 26.times.26 mm.sup.2. It should be noted that the
detection wavelength range centered on approximately 3000 cm.sup.-1
has an overlapping spectra with C--H stretching modes.
The infrared absorption characteristics by the example sample of
the manufactured structure 102 were measured. P-polarized light was
used as the irradiated infrared rays, the incident angle .theta.
was changed, and the reflection spectrum was measured by using
Fourier-transformed infrared spectrometer (FT-IR, JASCO, FT/IR-6300
FV) provided with a variable angle reflection accessory (Harrick,
Seagull). The optical path arrangement in the measurement is shown
in FIG. 3A. In order to improve the signal-to-noise ratio of the
infrared detection signal, inside of the sample container was
purged by dry nitrogen gas, and a high sensitivity MCT (HgCdTe)
detector which was cooled by liquid nitrogen was used at a
wavenumber resolution of 2 cm.sup.-1. The metal layer 34 on the
substrate 32 is a thick film of gold, which is thicker than the
skin thickness, and through which the infrared rays do not
transmit, and accordingly the incident infrared ray beam is either
reflected or absorbed by the surface structure. If the plasmon mode
is excited, the absorption dip appears in the reflection spectrum
due to the resonant action.
FIGS. 6A and 6B are reflectance maps showing the characteristics of
the infrared absorption of the structure 102 of the structure shown
in FIG. 3, as a function of frequency (or wavenumber; vertical
axis) and an incident angle (horizontal axis); and show the
experimental values (FIG. 6A) by the example sample and a numerical
simulation result (FIG. 6B) on the same condition as the example
sample, in a side-by-side manner. These reflectance values are
obtained after having been normalized by values obtained on an Au
reference sample. As shown in FIG. 6A, a shallow valley due to
several weak absorptions that depend on incident angles and a deep
valley due to three major absorptions which are the absorptions of
almost 100% were clearly observed at 1000 cm.sup.-1 to 5000
cm.sup.-1 (f=30 THz to 150 THz). FIG. 6B shows the result of a
series of numerical simulations carried out by using the finite
element method (FEM) in order to identify the plasmon mode which
becomes the cause of these absorptions. In the calculation, the
refractive index of SiO.sub.2 was set at 1.45, and the experimental
values were used for Au and MgF.sub.2. In the simulation result of
FIG. 6B, the excited plasmon modes are clearly shown by curves of
chain lines, which distinguish the resonance mode (m) from the
surface plasmon polaritons (SPPs) that are phenomena other than the
original intent.
In the reflectance map (FIG. 6B) by numerical simulation, the
experimental results shown in FIG. 6A were satisfactorily
reproduced in both qualitative and quantitative aspects. It can be
said that weak absorptions occurred due to Fabry-Perot resonances
in surface plasmon polaritons (SPPs) propagating on the
metamaterial surface of linear ribs, which is a one-dimensional
periodic surface structure. Specifically, the dispersion relation
generally satisfies the momentum conservation law of SPP
excitation, in other words, the relation of (w/c)sin
.theta.=(.pi./.LAMBDA.)I (.omega. is angular frequency, c is the
speed of light, and I is an integer). On the other hand, the
frequencies of the principal absorptions each labeled as m=1, m=2
and m=3 are irrelevant to the incident angle and originate in
plasmon resonance localized in Au/MgF.sub.2/Au structure. In this
way, it was confirmed that the surface of the example sample which
was actually manufactured structure 102 that was an array of the
linear ribs of the Au/MgF.sub.2/Au structure which the metal
component 50, the support part 40 and the metal substrate 30 form,
actually worked as the metamaterial absorber for the infrared ray
absorption surface 20.
Next, the electromagnetic field distributions (H.sub.y and E.sub.z)
of each mode based on the calculation are shown in FIG. 7. Each
mode corresponds to major absorption dips at .theta.=80.degree. in
FIG. 6B, respectively, and corresponds to m=1 at 1540 cm.sup.-1,
m=2 at 3013.3 cm.sup.-1, and m=3 at 4476.6 cm.sup.-1, in order from
FIGS. 7A to 7C. For the sake of the description, all the figures
are enlarged 3 times in the vertical direction. In correspondence
with the incident p-polarized light, the localized plasmon in which
a direction of the dipole is directed to a width direction of the
ribbon is resonantly excited in the metal component 50 that is the
upper Au ribbon. The localized plasmon of the metal component 50
induces its mirror image inside the metal substrate 30 which is a
thick Au film, and accordingly these images interact with each
other to generate plasmon hybridization. As a result, two new
specific modes which are symmetric and asymmetric modes are formed.
However, the symmetric mode is naturally prohibited because of the
parity due to mirror image interaction, and accordingly only the
asymmetric mode is selectively excited in this system. The
asymmetric mode that is referred to as a so-called magnetic mode
relates to the charge oscillation of opposite phase. Therefore, the
incident and re-radiated lights interfere destructively with each
other, which leads to effective cancelling out of the reflection
light. This physics in the metamaterial absorber suppresses
undesirable light reflection from the gold surface, and causes
strong light absorption.
The net electric dipole moment of the above described magnetic mode
becomes zero in the quasistatic limit, and accordingly the mode
excitation efficiency naturally decreases and strongly depends on
the retardation originating in oblique incidence. In the case of
m=2 (FIG. 7B), the electric field distribution is symmetrical as a
whole across the center of the metal component 50 which becomes the
surface structure, and accordingly such a mode cannot be ideally
excited in the vertically incident light (dark mode). However, in
the above mode, as the incident angle increases, the mode is
progressively excited due to the symmetry breaking in the x-axis
direction, and the dip of the absorption becomes clear at
.theta.>30.degree. (FIG. 6). A similar situation is observed in
the case of m=3, and the retardation effects cancel each other due
to the profile of the asymmetric mode, and the mode is not exited
at an incident angle of 40.degree. in particular.
Furthermore, in order to confirm the actual trace detecting
capability of the infrared ray absorption surface which achieves
both of the tailored plasmonic enhancement and the significant
suppression of the background, the resonant coupling was checked by
an experiment that adopts the infrared vibrational mode of
molecular self-assembled monolayer (molecular SAM) of the molecular
level as a coupling counterpart. Specifically, the resonant
coupling between the plasmonic mode of the structure 102 and the
molecular SAM was checked in the experiment.
FIG. 8A shows measured values of reflectance spectrum obtained from
SAM of 16-MHDA; and shows in order from the upper to the lower, the
measured values obtained at .theta.=80.degree. on a bare Au
surface, and measured values obtained at different incident angles
of 30.degree. to 70.degree. on the metal surface 36 of the
structure 102. As for the target molecules showing typical
symmetric and asymmetric C--H stretching vibrational modes at
approximately 2855 cm.sup.-1 and 2920 cm.sup.-1, the
16-Mercaptohexadecanoic acid (16-MHDA, made by Shigma-Aldrich)
depicted in the inset of FIG. 8A was adopted. The mode of m=2 in
the metamaterial absorber of the structure 102 is used for a mode
that forms a plasmon-molecule coupling system. This is because the
above mode of m=2 overlaps the above described symmetric and
asymmetric C--H stretching vibrational modes in the spectrum, and
it is appropriate to set the absorption band in the mode of m=2 as
the detection wavelength range covering the response wavelength of
the above described target molecule, or the substance to be
detected. The layer of the molecular SAM was prepared by using the
self-assembling capability which 16-MHDA has. As for a specific
process, it was determined that, firstly, the structure 102 was
immersed in a 16-MHDA ethanol solution of 10.sup.-3 M. After 48
hours, the sample was washed with ethanol and dried with dry
nitrogen gas to complete the preparation of an object to be
measured. Due to this process, the thiol head group was chemisorbed
onto the Au surface, and the whole of the structure 102 was covered
with SAM of the 16-MHDA having a thickness of 21.5 Angstrom (2.15
nm). As a reference measurement for a control, a comparative
example sample (bare Au sample) was prepared with the metal
substrate 30 of Au by adopting neither the support part 40 nor the
support part 40 while subjecting it to the same process.
It is to be noted in the upper part of FIG. 8A that the vertical
axis indicates a bright range where the reflectance of gold
appears, and the bar of 1 shows a range from 98% to 99%. In the
case of the bare Au sample, signals accompanied by fluctuation are
obtained, and it is helpful to identify a weak absorption spectrum
from among the noises due to the fluctuation. It follows that such
an operation suffers from an extremely low signal to noise ratio,
and that it leads a great difficulty in detecting each absorption
dip in the C--H stretching mode and determining the wavelength. On
the other hand, in the example sample of the structure 102 in the
lower part of FIG. 8A, Fano-like anti-resonance peaks (indicated by
upward arrow in the figure) are generated as a result of resonant
coupling between the plasmonic mode and the molecular vibrational
mode. Specifically, the structure 102 shows a wide plasmonic
absorption which becomes the detection wavelength range, at the
position of .omega..sub.pl=2921.9 cm.sup.-1. The dip of the
absorption became clearer as the increase of the incident angle
.theta.. Furthermore, the vibrational mode of the molecules in the
vicinity of the metal surface 36 of the structure 102 resonantly
coupled with the plasmonic mode of the structure 102. Peaks
generated in the wide absorption of the metamaterial are two
Fano-like anti-resonance peaks which are distinguished from each
other, and this absorption coupling process was dependent on the
incident angle. The clear vibration signal was not obtained when
the incident angle range was .theta.<30.degree. or
.theta.>70.degree., and the signal intensity became maximum in
the vicinity of .theta.=40.degree.. Concerning this reason, the
inventors consider that in the case of .theta.<30.degree., the
excitation of the plasmonic mode of the structure 102 has been too
weak to detect the anti-resonance peak; and that on the other hand,
in the case of .theta.>70.degree., the molecular vibrational
mode has been directly excited by incident infrared light, thereby
the absorption has increased, the competition has occurred between
the peak caused by the resonant coupling process and the above
absorption, and as a result, the signal has become weak.
The net value (vibrational signal) that excited the molecular
vibration can be extracted by performing a baseline correction of
dividing the measured reflection spectrum by the curve shape
obtained by plasmon resonance. The upper part of FIG. 8B shows thus
extracted vibrational signals at the incident angle
.theta.=40.degree.. Each vibrational signal in symmetric/asymmetric
C--H stretching modes was clearly observed. This result illustrates
our primary target of achieving metamaterial-enhanced infrared
absorption spectroscopy.
In order to quantitatively analyze the vibrational signal, Fano
curve-shape fitting was also performed according to the following
functional form of Formula 1 below:
.varies..omega..omega..times..times..gamma..omega..omega..gamma..times..t-
imes. ##EQU00001##
In Formula 1, .omega..sub.vib is a resonant frequency, .gamma. a
damping constant (FWHM), and F a Fano parameter for describing the
degree of asymmetry. The spectral curve shape of the experimental
results was satisfactorily reproduced by a Fano fitting curve using
corresponding parameters (lower part of FIG. 8B). The
.omega..sub.pl/.omega..sub.vib is detuning. The central frequency
of the fitting curve shows good coincidence with that of the
symmetric/asymmetric C--H stretching mode, and the fitting curve
could substantially accurately identify the specific functional
group. Due to the property of the Fano resonance, the fitting curve
was clearly changed from a symmetrical curve shape (F=0) to an
asymmetric one (F=-0.05) while being accompanied by frequency
detuning between the plasmon resonance and the molecular vibration.
By using a SAM packing density of 21.4 angstrom.sup.2/molecule
(0.214 nm.sup.2/molecule), the sensitivity in the case of the
infrared ray beam spot under the diffraction limit in FT-IR
reflection measurement is estimated, and the value was determined
to be approximately 1.8 attomoles.
As has been described above, a new spectroscopic technique based on
the metamaterial enhanced infrared absorption of the molecular SAM
was proposed and demonstrated. A low background detection technique
was demonstrated which was attended with the tailored plasmonic
resonance by a metamaterial absorber such as the structure 102, and
the sensitivity of the attomole level was achieved in direct
infrared absorption spectroscopy. A noteworthy fact in the present
embodiment is that a sensitivity of the attomole (10.sup.-18 moles)
level was achieved at the high signal-to-noise ratio in the
far-field measurement. In the principle as well, we confirmed that
the resonant interference of the plasmon-molecule coupling system
which becomes the cause actually occurred, through a spectrum
analysis using the Fano curve-shape fitting. This result shows a
direct evidence of a fact that in addition to the resonance between
the light and the structure, the resonance between light and
molecular vibration actually occurs, and that both resonances
interact with each other. The low background originating in the
above interaction and the peak at the reaction wavelength in the
background bring a new principle to the infrared spectroscopy, open
a way to an ultra-sensitive infrared inspection technology, and can
be an evidence that the technique using the metamaterial of the
present embodiment can be new means of achieving the
ultra-sensitive infrared inspection technology.
5. Modified Example
The present embodiment of which the possibility has been checked in
the above described example can be also implemented after having
received various modifications as will be described below.
5-1. Modification of Linear Rib Structure
The structure of the present embodiment can be modified in order to
adjust the wavelength region and the wavelength width for the
detection wavelength range, or to adjust the angle at which the
peak becomes most clear that originates in the molecular vibration
of the substance to be detected, among the incident angles (FIG. 1,
.theta.) of the infrared rays. In the case where the structure 102
is adopted in which the metal component 50 is a linear rib
structure as shown in FIG. 3, the detection wavelength range by the
plasmonic absorption can be changed shorter by shortening the
period .LAMBDA.. Also as for the support part 40, various
structures other than the above described support part 40 can be
adopted as long as the support part supports the metal component 50
relative to the metal substrate 30. For instance, the structure of
the support part 40, which is configured to cover the whole surface
of the metal substrate 30 so as to support the metal component 50,
can be said to be advantageous in a point that it is easy to
manufacture. The distance separating the metal component 50 from
the metal substrate 30 affects the detection wavelength range. The
wavelength region and the wavelength width of this detection
wavelength range can be easily adjusted by the thickness of the
support part 40, and the like.
5-2. Modification of Resonator
In the new infrared spectroscopy by the inventors, in addition to
the above described adjustment of size and selection of material, a
device structure, a designing method and the manufacturing process
which have been described in the prior report by the present
inventors can be also adopted, for the purpose of performing the
concept of the present disclosure. For instance, preferable
examples in the present embodiment include: the metal component 50
that forms a ring which has a two-dimensional planar shape, and
that forms a split-ring similar to a ring having a gap at least in
a part in the circumferential direction (for instance a C-shape).
Split-ring resonators (SRRs) such as a single split-ring resonators
(SSRRs) and a double split-ring resonators (DSRRs), in particular,
will exhibit sufficient performance to eliminate unnecessary noise
in spectroscopic application. The reason is because the absorber
having the split-ring resonator has a capability of adapting the
wavelength region to a desirable one with higher accuracy, by being
tailored according to our designing technique. Specifically, it is
possible to use a resonator in which a metal ribbon or wiring forms
a ring shape or a resonator having the above described split ring
(Non-Patent Literature 4), in place of the resonator by combination
of the metal component 50, the support part 40 and the metal
substrate 30 in FIG. 3A. FIG. 9A shows an example in which an array
of SSRR 60 is formed for the infrared ray absorption surface 20. In
this array, the resonant action is achieved by mainly magnetically
coupling infrared rays (light) with each SSRR 60, and the infrared
ray absorption surface results in showing the absorption in the
detection wavelength range. By determining the size of each part of
the SSRR 60, it becomes possible to tailor the infrared ray
absorption surface so as to match the target detection wavelength
range for infrared rays and a polarized light to be used for
measurement. Specifically, in order to adjust the resonant action
of the resonator by the ring or the split ring, generally, sizes
such as the inner diameter and the outer shape of the ring, and the
width of the ring portion are adjusted. For the split ring, the
size of the gap and further the thickness are also adjusted.
5-3. Three-Dimensional Ring
Furthermore, the split-ring resonator that can be applied to the
present embodiment is not limited to only the split-ring resonator
having a planar shape along the infrared ray absorption surface.
For instance, a 3D-SRR (three-dimensional split-ring resonator) 62
which is shown in FIG. 9B has been proposed by the present
inventors, in which each of the resonators rises from the surface
including the array of the resonator, and becomes a resonator
having a portion extending to a deviating direction at least in the
part thereof (Non-Patent Literature 5). Such a 3D-SRR 62 can be
formed from a ribbon-shaped pattern of a metal layer, which has
been once patterned onto a plane by a lift-off or CF.sub.4 process
or the like, and by such a process (self-folding process) that the
pattern is folded by itself. Such a shape and a manufacturing
technique can also be an example of the structure of the present
embodiment.
5-4. Modification of Metal Type
In addition, at the time when each of the above described
resonators is designed, it is possible to adapt the structure of
the present embodiment to the substance of interest to be detected
on the basis of our theoretical approach, by tailoring the
detection wavelength range in consideration of the electrical
and/or magnetic response of the metal type such as Au, Ag, Cu, Al,
and Pt. The present inventors have reported general effects of
changing the metal material on the characteristics of the
metamaterial, in the non-patent Literature 4. For instance, the
inventors have analyzed the behavior of the SSRR of a type having
two gaps in each ring, by correctly describing the behavior in the
optical frequency region including the visible region of the metal.
This analytical technique which adequately reflects the properties
of the metal can also be applied to the material for the metal
component 50 and the metal surface 36, and is also useful for
accurately predicting other types of metamaterials such as linear
ribs and the actions in the infrared region.
5-5. Candidate of Substance to be Detected
As for substances to be detected by the structure of the present
embodiment, substances of various materials and properties are
considered. In the above described example, 16-MHDA has been
selected as the material to become the self-assembled film, but
this is an exemplification for checking the feasibility of the
structure of the present embodiment and infrared spectroscopy using
the structure, by the substance to be detected of which the
molecular number is easily estimated. In order to implement the
present embodiment, the structure is adjusted to adapt the
detection wavelength range to the response wavelength of the
substance to be detected. In the case of implementation by the
structure 102 (FIG. 3), the shapes, sizes, and materials of the
metal substrate 30, the support part 40, and the metal component 50
are determined according to the substance to be detected, in
consideration of modes (stretching mode, bending mode and the like,
and symmetry of vibration) that will provide for the molecular
vibrations to be detected.
Thus, in the present embodiment, the candidate which can be the
substance to be detected is not particularly limited. In other
words, arbitrary organic and inorganic substances that can be
objects of infrared spectroscopy generally become substances to be
detected irrespective of those physical properties. In other words,
the substances to be detected are all such substances as to
generate a phenomenon based on the response (for instance,
molecular vibration) of the substance involved in the resonant
phenomenon with the electromagnetic field, in the infrared
wavelength range (far-infrared to mid-infrared region or THz wave
region). Therefore, the substance to be detected includes all
substances which are an object of the conventional infrared
spectroscopy. There is not particular restriction on the specimen
itself which potentially contains the substance to be detected. As
for non-limited examples, the substances to be detected include
O.sub.2, HF, CH.sub.4, H.sub.2S, NO, NH.sub.3, CO.sub.2, CO,
N.sub.2O, CH.sub.4, H.sub.2O, SO.sub.2, SO.sub.3, NO.sub.2,
SO.sub.2, acetone, aromatics, sugars (such as glucose), SF.sub.6
and ethylene, and arbitrary known substances such as a monomer, a
dimer, an oligomer, a polymer, proteins and nucleic acids, which
have the response wavelengths corresponding to arbitrary
vibrational modes of all the bonds contained in the substances
become objects to be detected. In addition, specimens that
potentially contain the substance to be detected include known
substances having arbitrary properties, such as a liquid, a gas, a
solid, a gel and a sol which potentially contain the substance in a
part of the components. In addition, as long as the substance
responds to the infrared rays, it can be said that an arbitrary
substance containing an unknown substance can also be the substance
to be detected.
Even when any substance to be detected becomes the object, it
becomes possible to improve the sensitivity to the Zeptomole level,
by optimizing the surface structure so as to increase the
overlapping of the detection wavelength range given by the
absorption of the infrared rays due to the plasmon mode with the
response wavelength mode of the molecular vibration.
5-6. Intermediate Layer and Intermediate Space Between Infrared Ray
Absorption Surface and Substance to be Detected
In the present embodiment, an example has been described in which
the specimen mainly comes in direct contact with the infrared ray
absorption surface, but as long as the peak originating in the
substance to be detected is found in the infrared reflection
spectrum as described in FIG. 2, the substance to be detected does
not necessarily need to come in direct contact with the infrared
ray absorption surface. For instance even though any layer or space
is intentionally or unintentionally inserted between the infrared
ray absorption surface and the substance to be detected, if the
infrared ray absorption surface and the substance to be detected
come into close contact with each other through such layer or
space, there is a possibility that the substance can be detected by
taking advantage of the high sensitivity and signal-to-noise
ratio.
5-7. Plurality of Types of Patterns and Plurality of Wavelengths
that are Matched to One Type of Substance to be Detected
The structure 100 of FIG. 1 and the method of the infrared
spectroscopy shown in FIG. 4 can be modified from various
viewpoints, mainly, from the viewpoint of practicality. The
substance to be detected generally shows one or more response
wavelengths. Let the response wavelengths be the first and second
response wavelengths which are different from each other. If the
first and second response wavelengths are closely located, one
detection wavelength range D can cover both of the wavelengths (for
instance, FIG. 8). On the other hand, there is also a case where
the first and second response wavelengths are too distant from each
other to be covered by a single detection wavelength range D. In
that case, if the detection wavelength range is set so as to
include the first detection wavelength range which includes the
first response wavelength and the second detection wavelength range
which includes the second response wavelength, it becomes possible
to perform analysis matching the substance to be detected.
FIGS. 10 and 11 are schematic views each showing such a
configuration. A structure 104 having a plurality of types of
groups of metal components typically has an infrared ray absorption
surface 24-1 in which a first group of metal components for a
detection wavelength range D1 are aligned in the infrared ray
absorption surface 24, and an infrared ray absorption surface 24-2
in which a second group of metal components for a detection
wavelength range D2 are aligned in the absorption surface 24, which
are distinguishably formed (FIG. 10). Thereby, it becomes possible
to realize an intensity spectrum which should be acquired when
analyzing the substance to be detected, by such a simple operation
of a level as to change an irradiation position on a piece of an
object structure, as needed. In the case where the linear rib
structure (FIG. 3) is adopted in which the ribbons are aligned, the
infrared ray absorption surface 24-1 and the infrared ray
absorption surface 24-2 can achieve detection wavelength ranges
that fit for the response wavelengths respectively, by changing,
for instance, a width w and a period A of the metal component
50.
Moreover, in another typical example, as in the structure 106
having a plurality of types of groups of metal components shown in
FIG. 11A, the infrared ray absorption surface 26 has such a
plurality of types of metal components as an infrared ray
absorption surface 26-1 for a detection wavelength range D1 and an
infrared ray absorption surface 26-2 for a detection wavelength
range D2, which are overlapped in the thickness direction. In the
structure 106 of FIG. 11A, the two infrared ray absorption surfaces
26-1 and 26-2 are shown separately from each other for explanation,
but may actually come into contact with each other or be integrated
with each other. The infrared ray absorption surface 26-1 and the
infrared ray absorption surface 26-2 which have been overlapped in
such a way can be also achieved also by adopting the liner rib
structure (FIG. 3) in which the ribbons are aligned. For instance,
as shown in FIG. 11B, the structure employs a first type of metal
component 52 and a second type of metal component 54 which have
different widths w of the metal component 50, for instance. In this
case, the infrared ray absorption surface 26-1 is achieved by a
resonator which is formed of the metal component 54, a support part
44 and the metal layer 34. In addition, the infrared ray absorption
surface 26-1 is achieved by a resonator which is formed of the
metal component 52, a support part 42 and the metal component 34.
Such a structure 106 can absorb infrared rays in both of the
detection wavelength range D1 and the detection wavelength range D2
(FIG. 11A), only by using a piece of structure and using only one
infrared beam for infrared spectroscopy.
The structures 104 and 106 illustrated in FIGS. 10 and 11 are
especially useful for such a substance to be detected as to be
easily detected if the substance is determined by a combination of
a plurality of response wavelengths that resist being covered with
only one detection wavelength range. In addition, also in the case
where the substance to be detected itself is a mixture of a
plurality of types of substances and shows a plurality of response
wavelengths, the structures 104 and 106 are useful.
In the infrared spectroscopy adopting the structures 104 and 106,
in place of the determination step S08 out of the processes shown
in FIG. 4, a step of determining at least any of a
presence/absence, a component content, a component type, a chemical
structure and ambient information regarding the substance to be
detected in a specimen is carried out, on the basis of reflection
peaks of a plurality of response wavelengths (first and second
response wavelengths) corresponding to the substance to be
detected, which appears in the reflection spectrum of the infrared
rays.
In the case where the substance to be detected showing a plurality
of response wavelengths such as the first and second response
wavelengths is determined to be the object in the present
embodiment, it is also useful to use not a piece of structure but a
plurality of pieces of structures which are manufactured so as to
match each of the wavelengths. Briefly, it is assumed to have
adopted a first structure having a first infrared ray absorption
surface and a second structure having a second infrared ray
absorption surface. At this time, the first and second infrared ray
absorption surfaces are manufactured so as to absorb the infrared
rays of the first and second detection wavelength ranges,
respectively, and the first and second detection wavelength ranges
are set so as to contain the first and second response wavelengths,
respectively. In a typical case where such a plurality of pieces of
structures are used, firstly a specimen that potentially contains
the substance to be detected is brought into close contact with
both of the infrared ray absorption surfaces of the first and
second structures. Then, infrared rays having a wavelength range
covering both the first detection wavelength range and the second
detection wavelength range are operated so as to irradiate the
first infrared ray absorption surface. Furthermore, the reflected
infrared rays reflected from the first infrared ray absorption
surface are operated so as to irradiate the second infrared ray
absorption surface. Finally, the reflected infrared rays reflected
from the second infrared ray absorption surface are detected by the
detector. If a plurality of pieces of separate structures for
different response wavelengths are combined in this way, the
practicality of infrared spectroscopy can be improved, which is
based on the plurality of response wavelengths of the substance to
be detected. Incidentally, the first and second response
wavelengths may originate in separate chemical bonds contained in
the substance to be detected, or in separate modes of one chemical
bond. Thereby, it becomes possible to acquire information on the
plurality of response wavelengths with single measurement, and to
detect a target substance to be detected while distinguishing the
target substance from another substance having a similar response
wavelength.
6. Summary
In the above, the embodiments of the present disclosure have been
specifically described. The above described embodiments and
structural examples have been described for explaining the
disclosure, and the scope of the present disclosure should be
determined on the basis of the description of the claims. In
addition, modified examples existing within the scope of the
present disclosure, which include other combinations of each of the
embodiments, are also included in the scope of the claims. Some
technology contents of the embodiments of the present disclosure
have been disclosed in detail in non-patent Literatures 4 and 5,
among the preceding reports by the present inventors, to such an
extent that those skilled in the art can carry out, and the
disclosed contents are quoted as they are and thereby shall be
incorporated herein to the present specification.
INDUSTRIAL APPLICABILITY
The present disclosure can be used for such arbitrary devices as to
detect, quantify or identify substances with the use of infrared
rays.
The various embodiments described above can be combined to provide
further embodiments. All of the U.S. patents, U.S. patent
application publications, U.S. patent applications, foreign
patents, foreign patent applications and non-patent publications
referred to in this specification and/or listed in the Application
Data Sheet are incorporated herein by reference, in their entirety.
Aspects of the embodiments can be modified, if necessary to employ
concepts of the various patents, applications and publications to
provide yet further embodiments.
These and other changes can be made to the embodiments in light of
the above-detailed description. In general, in the following
claims, the terms used should not be construed to limit the claims
to the specific embodiments disclosed in the specification and the
claims, but should be construed to include all possible embodiments
along with the full scope of equivalents to which such claims are
entitled. Accordingly, the claims are not limited by the
disclosure.
REFERENCE SIGNS LIST
100, 102, 104 and 106 structure
20, 22, 24, 24-1, 24-2, 26-1 and 26-2 infrared ray absorption
surface
30 metal substrate
32 substrate
34 metal layer
36 metal surface
40, 42 and 44 support part
50, 52 and 54 metal component
60 single split-ring resonator (SSRR)
62 3D split-ring resonator (3D-SRR)
* * * * *